Answer:
70.07 Hz
Explanation:
Since the sound is moving away from the observer then
and 
when moving towards observer
With 
of 76 then taking speed in air as 343 m/s we have
Similarly, with of 65 we have
Now
v_s=27.76 m/s
Substituting the above into any of the first two equations then we obtain
The answer is C. The mass of the platinum sample is greater than the mass of the lead sample. As I explained in a previous answer, if they are the same volume, but one is heavier, then it must be more dense. In this particular example, the platinum is more dense than the lead, and therefore has more mass.
-- A motor and a generator both do a transformation between electrical energy and some other form of energy, but they do it in opposite directions.
-- A motor takes electrical energy and transforms it into mechanical energy, which can then be used to run mechanical things like cars or wheat grinders.
-- A generator takes mechanical energy ... like from a steam turbine or a windmill or a water wheel ... and transforms it into electrical energy, which can then be shipped over long distances through wires, and used to run motors or other electrical things.
